Today, magnetostrictive multi-layer systems, such as, for example, spin-valve systems, are employed as sensors in various ways. The document K. Ludwig et al. “Adapting GMR sensors for integrated devices” Sensors and Actuators A, 106, 2003, pp. 15-18, for example, describes an application of GMR sensors in integrated devices as current sensors and magneto-couplers. The GMR magneto-couplers comprise a coil layer where a signal current is converted into a magnetic field. By applying a magnetic field by means of a current in the coil layer, the output signal of the magneto-coupler is shifted to positive values due to ferromagnetic coupling when the magnetic field applied is along the direction of the magnetization of the magnetically hard layer. The shift may be compensated by arranging four sensors in a bridge structure so that a sensor output characteristic can be adjusted or designed by designing geometrical arrangements correspondingly. Furthermore, WO/95/28649 shows a magnetic field sensor where an auxiliary magnetic field is generated to adjust an operating point of the magnetic field sensor. The auxiliary magnetic field can be generated by an electrical conductor, wherein in feedback operation a voltage of the magnetic field sensor elements is kept constant and the current through the electrical conductor is measured.
Lately, ever-increasing fields of application have developed in sensor technology for magnetoresistive GMR/TMR multi-layer systems (GMR/TMR=giant magneto resistance/tunneling magneto resistance) due to their extraordinary characteristics with regard to sensitivity and structural sizes required. Spin-valve systems on the basis of GMR systems or TMR systems principally comprise a layer setup where a magnetically hard layer the magnetization of which is pinned is arranged above or below a magnetically soft layer the magnetization of which may be adjusted freely in an external magnetic field. The magnetically hard layer and the magnetically soft layer are separated from each other by a non-magnetic layer which, in a GMR system, is a non-ferromagnetic metal layer and, in a TMR system, includes a non-metallic insulating layer. Magnetostrictive GMR/TMR structures can be employed as extensometers at micromechanical bending beams or movable membranes for detecting accelerations or pressure. It is of advantage in such structures for the free layer to be formed of a material comprising high magnetostriction to obtain the highest sensitivity possible.
When manufacturing such sensors, deviations caused by the manufacturing, however, must be expected, such as, for example, due to the dimensions and thickness of the micromechanical bending beams or of the movable membranes, affecting the mechanical sensitivity of the sensor.
Furthermore, deviations of the magnetic characteristics of the active GMR/TMR layers result, such as, for example, of the relative position of the magnetizations or of the anisotropy field strengths having an effect on the magnetostrictive sensitivity. The deviations mentioned above inevitably result in yield losses with a predetermined product specification, considerably increasing the costs for manufacturing spin-valve-based sensors in industrial production.
Apart from the difficulties of a cheap manufacturing of spin-valve-based sensors, these systems also entail operating difficulties.
Since the magnetostrictive effect is a non-linear effect, the output signal of well-known sensors can only be described roughly in a small range by means of a first-order function. Consequently, it is not possible to obtain a linear output signal over the entire operating range of the sensor without performing complicated subsequent correctional calculations of the output signals. Apart from the fact that the additional provision of calculating capacities of this kind results in an enlargement of the devices or blocks calculating capacities present, it is necessary with certain applications to provide output signals with high precision without delay, such as, for example, in acceleration sensors employed in the automobile sector. Using complicated calculations here is not only frequently of disadvantage but sometimes even impossible.
Consequently, it would be desirable to keep the cost and complexity of the evaluating electronics in a sensor as small as possible, i.e. to obtain a reproducible output characteristic field having the highest linearity possible, such as, for example, a linear extension/resistance output characteristic field.
Furthermore, adjusting the operating point of the sensitivity entails great problems, in particular because a reproducible sensitivity is not possible for certain operating points of well-known sensors. In addition, the sensitivity of well-known spin-valve sensors is not constant over the entire extension range, wherein additionally saturation effects occur, resulting in a flattening of the output characteristic curve and thus strongly limiting the operating region. This might have the effect that sensors cannot be used for certain applications where a deformation must be detected over a wide range, such as, for example, in acceleration sensors subjected to high accelerations.
With regard to a performance of GMR/TMR structures under mechanical extension, the document M. Lohndorf et al. “Strain Sensors based on magnetostrictive GMR/TMR structures”, IEEE Transaction on Magnetics, Vol. 38, No. 5, 2002 discloses results of measurements with GMR multi-layer systems and MTJ systems where a mechanical extension is caused by a bending apparatus. A magnetic field was applied during the measurements to record the course of the resistance as a function of the magnetic field. Due to the higher MR ratio (magneto-resistance ratio), the usage of MTJ systems in magnetostrictive applications is thought of as being of advantage.
In addition, the document M. Lohndorf et al. “Highly sensitive strain sensors on magnetic tunneling junctions”, Appl. Phys. Letter, Vol. 81, No. 2, 2002, pp. 313-315 describes measurements with MTJ systems where a magnetic field is applied in a first configuration in parallel to the magnetization axis of the MTJ system and the direction of an extension applied. In a second configuration, the extension applied is applied perpendicularly to the magnetic field applied and the magnetization axis of the MTJ system. For the first configuration in parallel to the magnetization axis of the MTJ system, the occurrence of hystereses can be observed, whereas considerably reduced hysteresis effects can be observed with the second configuration of a perpendicularly applied extension.